Sea State Report Lincolnshire

Transcription

1 Sea State Report Lincolnshire Year 3 and summary for October 2006 September 2009 RP027/L/2012 November 2012 Title here in 8pt Arial (change text colour to black) i

2 We are the Environment Agency. We protect and improve the environment and make it a better place for people and wildlife. We operate at the place where environmental change has its greatest impact on people s lives. We reduce the risks to people and properties from flooding; make sure there is enough water for people and wildlife; protect and improve air, land and water quality and apply the environmental standards within which industry can operate. Acting to reduce climate change and helping people and wildlife adapt to its consequences are at the heart of all that we do. We cannot do this alone. We work closely with a wide range of partners including government, business, local authorities, other agencies, civil society groups and the communities we serve. Published by: Shoreline Monitoring Group Environment Agency Kingfisher House, Goldhay Way Orton Goldhay, Peterborough PE2 5ZR enquiries@environmentagency.gov.uk Environment Agency 2012 All rights reserved. This document may be reproduced with prior permission of the Environment Agency. Further copies of this report are available from our publications catalogue: or our National Customer Contact Centre: T: E: enquiries@environment-agency.gov.uk. ii Sea state report Lincolnshire ACMVII

3 Anglian Coastal Monitoring Sea state report Lincolnshire ACMVII iii

4 Contents 1 Anglian Coastal Monitoring (Phase 7) Wave and tide monitoring Sea state reports Acoustic Wave & Current meter Directional Waverider Mark 2 wave buoy Tidal regime Instrument data return Instrument locations 4 2 Wave statistics Monthly and annual means Significant wave height Wave direction Peak period Period and significant wave height Wave energy Wave return periods 18 3 Temperature 20 4 Sea level & tides 21 5 Storminess & extremes Storm wave events Monthly wave maxima Monthly temperature maxima Highest Lincolnshire storm event 2008/ Surge events Highest Lincolnshire surge events 2008/ Storm calendars Maximum sea level 28 6 Summary 29 Table 1: Deployment locations between of Lincolnshire instruments. 4 Table 2: Monthly mean significant wave heights Hs values (m) in Year 3. 6 Table 3: Annual mean significant wave heights Hs values (m) for all years. 6 Table 4: Monthly mean maximum wave heights Hmax values (m) in Year 3. 6 Table 5: Annual maximum significant wave heights Hs values (m) for all years. 7 Table 6: Monthly mean peak wave period (Tp) values (s) in Year 3. 7 Table 7: Annual mean peak wave period (Tp) for all years. 7 Table 8: Monthly mean wave period (Tz) values (s) in Year 3. 7 Table 9: Annual mean wave period (Tz) values (s) for all years. 8 Table 10: Monthly mean wave direction (Mdir) values ( ) in Year 3. 8 Table 11: Annual median of main wave direction (Mdir) values ( ) for all years. 8 Table 12: Return period for S1L AWAC ( ). 18 Table 13: Return period for S2L AWAC ( ). 18 Table 14: Return period for S3L AWAC ( ). 19 Table 15: Return period for S4L AWAC ( ). 19 Table 16: Monthly mean temperatures. October 2008 September Table 17: Monthly mean sea levels relative to Ordnance Datum Newlyn for Year 3 (October 2008 September 2009).21 iv Sea state report Lincolnshire ACMVII

5 Table 18: Tidal parameters. 21 Table 19: Monthly maximum significant wave height (Hs) values(m) 23 Table 20: Monthly maximum wave heights (Hmax) values (m) 23 Table 21: Monthly maximum peak wave period values (s) 24 Table 22: Monthly maximum mean wave period values (s) 24 Table 23: Monthly maximum temperature ( c). 24 Table 24: The three highest storm wave (Hs) events. 25 Table 25: Surge events in Year Figure 1: Map of instrument locations vi Photo 1: AWAC in frame being deployed 3 Photo 2: Deployed Waverider buoy 3 Figure 2: Map showing location of Lincolnshire instruments 5 Figure 3: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S1L at Donna Nook (blue) and the West Silver Pit wavebuoy (LWB1). 9 Figure 4: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S2L at Theddlethorpe (light blue) and the West Silver Pit wavebuoy (LWB1). 10 Figure 5: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S3L at Chapel Point and the West Silver Pit wavebuoy (LWB1). 10 Figure 6: Plot of significant wave height (Hs) records from October 2006 to September 2009 for the AWAC S4L at Skegness and the West Silver Pit wavebuoy (LWB1). 11 Figure 7: Main wave direction (Mdir) and occurrences for Lincolnshire AWACs and the offshore waverider buoy, from October 2006 to September Figure 8: A distribution plot of wave periods for each AWAC from Figure 9: Peak wave periods recorded by LWB1 West Silver Pit from September 2006 October Figure 10: Scatter plots indicating joint distribution of Hs and wave period for October 2006 September 2009 at each AWAC site 15 Figure 11: Wave spectra plot showing for S3L at Chapel Point for the month of November Figure 12: Wave energy and wave period distribution over frequency for all years ( ), 16 Figure 13: Comparison of the average wave energy (Tz) at each AWAC site for each year ( ). 17 Figure 14: Comparison of the peak wave energy (Tp) at each Lincolnshire AWAC for each year ( ) 17 Figure 15: Lincolnshire monthly mean temperatures. 20 Figure 16: Sea level at each AWAC (S1L, S2L, S3L, S4L) for October 2008 September Figure 17: The November surge event, 26 Figure 18: Storm calendar for October 2006 September Figure 19: Surge calendar for October 2006 September Figure 20: Maximum sea levels recorded at each AWAC in each year. 28 Sea state report Lincolnshire ACMVII v

6 Figure 1: Map of instrument locations vi Sea state report Lincolnshire ACMVII

7 1 Anglian Coastal Monitoring (Phase 7) 1.1 Wave and tide monitoring The Shoreline Monitoring Group (SMG) based within the Regional Flood & Coastal Risk Management department of the Environment Agency (EA) (Anglian Region) provides strategic monitoring of the Anglian coast through the Anglian Coastal Monitoring (ACM) programme. Phase VII of this programme ran from 2006/07 to 2010/11. For wave and tide monitoring, a network of five Directional Waverider (DWR) buoys and 20 Acoustic Wave and Current meters (AWACs) measured offshore and nearshore sea conditions respectively, along the regional frontage over a three year period ( ). The AWACs were recovered in 2009, the West Silver Pit wavebuoy was later recovered due to its proximity to the wavebuoy deployed at Inner Dowsing. The remaining four DWR buoys are now managed either by the SMG or the UK Coastal Monitoring and Forecast service (UKCMF). This has allowed the buoys to remain in-situ, and for continued wave recording. The wavebuoys provide real-time wave spectra and GPS positions via satellite link. The real-time wave data is uploaded to the WaveNet 1 website maintained by the Centre for Environment, Fisheries and Aquacultural Science (Cefas), on behalf of the Department of Environment Food & Rural Affairs (Defra), allowing public access to the programme s real-time monitoring data. The data collected over the three year period has proved to be of significant value. It has facilitated valuable observations of the Anglian coastal sea state which was previously only modelled. In addition, following recovery of the AWACs, coincidental logging of offshore and nearshore data has allowed development of statistical models that provide inshore wave conditions calculated from offshore buoy observations. These models have been used to create look up tables for use in flood forecasting. The Anglian network is now part of a national programme supplying real time data to the UK Meteorological Office to be integrated into the Environment Agency s National Flood Forecasting System and wave overtopping models. In the next phase of monitoring the SMG plans to expand the buoy network, deploying three further buoys along the Anglian coast. It is envisaged that in time, the data will continue to provide input into wave models and help to build a dataset for statistical forecasting of extreme conditions to assist in flood forecasting and flood risk management. It is also hoped that the wave and currents data may be combined with sediment models to identify the transportation and movement of sandbanks, annual beach sediment movement and erosion following storm events, and the impact of sea level rise throughout the region. Furthermore, the data will provide baseline design statistics to inform future coastal and marine planning, construction projects, and further climate research and monitoring of sea level rise. The data is already being used to increase our understanding of coastal processes and forecasting beach evolution regarding beach erosion, sediment transport and geomorphological trends enabling operational assistance in projects such as beach renourishment schemes. Further to this, it is hoped that the data will be used to link storm events and wave conditions with impacts on the coastline and to assets, thereby aiding development of a tool to trigger actions such as post storm monitoring or emergency works. We will continue to provide this data on the WaveNet website making it available to the public and of use to recreational coastal users e.g. leisure, boating, surfers and divers. The dataset also provide an opportunity for further research such as joint probability studies, identifying extreme offshore and nearshore water levels, wave transformation, tidal propagation, current behaviour and coastal response studies. Sea state report Lincolnshire ACMVII 1

8 1.2 Sea state reports This report will be of interest to coastal managers that undertake, or provide input to, strategic planning, capital engineering works and maintenance programmes. In addition, the reporting of sea state will be of assistance with general education and awareness raising of coastal issues. This report is the third regional sea state report and it provides a summary of data gathered between It follows on from the previous sea state reports for the region which provide a more in depth discussion of the Lincolnshire coast, wave and extremes studies in addition to the annual datasets and analysis (see Environment Agency, 2009; 2010). The data used in this report is the processed on-board logged data from each of the four AWAC instruments and one offshore buoy located off the Lincolnshire coast. The logged data was processed and quality checked by Gardline Environmental 2. The QA process for the AWACs uses Nortek s control software 3 and includes compass, pressure drift, acoustic degradation checks and tide level adjustment to Ordnance Datum (Newlyn). There are further adjustments for atmospheric pressure, accounting for instrument settling periods, such as frame settlement on the bed and an initial temperature offset and differences in AWAC deployment periods. The wave buoys are fully calibrated before deployment and data translation and processing is carried out using the instrument manufacturer s software including Datawell s W@ves The three-year monitoring period is not sufficient to draw long term conclusions regarding sea and climate trends. Although this report assesses changes in sea state from previous two reports, the main purpose is an assessment of the current sea state influencing the Anglian coast. The report presents the monitoring data to show the types of waves and the seasonal variation over the year and thus gives a picture of the wave climate. The AWAC instruments record the nearshore climate of waves and currents that are complicated by bed topography at the coastline while the wave buoys give an indication of the waves further offshore and approaching the region s shoreline. Knowledge of the Anglian wave climate is important to determine and model regional movement of sediment and the impact of forces acting on our sea defences, coastal structures and habitats. 1.3 Acoustic Wave & Current meter The 20 AWAC instruments were placed at strategic locations distributed along the five Shoreline Management Plan extents on the Anglian coast. The instrument sits within a frame on the sea bed at a depth of approximately 6 m Chart Datum (CD) with sensors pointing up towards the surface and recording tidal elevations, waves, currents and surge information. The AWAC is a current profiler with a directional wave system. The instrument measures current velocity and direction at different bin depths throughout the water column. The AWAC has three sensor pads which emit a pulse acoustic signal in different directions radiating out from the instrument towards the surface. The scattered return signal has a Doppler shift with respect to the transmitted signal and this allows the along-beam velocity to be calculated. The instrument also has a central sensor pointing vertically at the sea surface, similar to an upward looking echo-sounder, the Acoustic Surface Tracking feature of the instrument gives water depth and non-directional spectrum measurements. The number of measurements allows the instrument to calculate the velocity and direction of currents throughout the water column, wave direction, heights and identification of long swell waves, wind waves, ship wake, pressure and temperature. The AWACs sample pressure, temperature, currents and acoustic back scatter intensity (ABSI) every 5 minutes at 20, 25, 30, 35, 40, 45, 50 and 55 minutes past each hour. Waves are sampled every hour, with 2048 samples taken at 2 Hz over 17 minutes. The instrument has a maximum error of +-50 mm for water levels, +5% for waves. However we consider a consistent achievable height accuracy to be 10 mm. AWAC instruments and the onboard logged data were recovered approximately every 6 weeks, with a new instrument redeployed at this time. There were 12 backup AWACs, used to cycle the instruments or as emergency standbys in the event of instrument failure. This allows the Shoreline Management Group a near uninterrupted and continuous record of waves 2 Sea state report Lincolnshire ACMVII

9 and tides as they enter the shallow waters of our coast. Photo 1: AWAC in frame being deployed (photo: Gardline Environmental) 1.4 Directional Waverider Mark 2 wave buoy The DWR buoys provide real-time information on waves approaching the Anglian coast. The buoys are moored to the seabed by an elasticated line allowing them to float on the surface and record wave movements. Similar to the AWAC, the buoys measure the orbital motions of the water at the surface rather than the surface slope. These continuous measurements are then sent ashore through high frequency radio signals to base stations at RNLI Life Boat stations. The SMG can then monitor and log these data streams through a broadband internet connection. Wave spectra and GPS positions are also sent via satellites. The real-time wave data is available on the WaveNet 1 website allowing public access to the programme s wave buoy data. Every 30 minutes the DWR logs processed spectral data of 2304 samples measured over a 19.2 minute period. The logged spectra have a 64 frequency band energy density resolution. The first 30 minutes of each hour is processed and quality checked to give a representative value of the hour, and is used as comparison against the hourly AWAC data which is sampled over the first 17 minutes of each hour. DWR buoys are serviced approximately every 6 months to check the buoy condition and to retrieve the onboard logged data. There are also two back up buoys ready for deployment. Photo 2: Deployed Waverider buoy (photo: Environment Agency) Sea state report Lincolnshire ACMVII 3

10 1.5 Tidal regime The flood tidal current generally flows south and ebb tidal current flows north along the Lincolnshire coast (Haskoning, 2005). Along the coast, wave action dominates but the tidal current residuals become more significant in the nearshore and offshore. These currents regulate sediment transport and the development of sand banks in the area, such as the Skegness Middle. Interaction between the tidal currents and sandbanks creates areas of turbidity known as overfalls. The Saltfleet, Theddlethorpe, Trusthorpe and Inner Dowsing Overfalls modify the waves approaching the Lincolnshire coast and the S2L, S3L and S4L AWAC instruments. 1.6 Instrument data return The ACM programme phase VII ZERO HOUR was designated 00:00h (GMT) 20 th September 2006 (Julian Day 263). Instruments were deployed from this date, and Year 3 of the project is defined as the period from 1st October 2008 to 30th September Lincolnshire was also previously monitored during 20th September th September 2006 as part of ACM phase VI. At Donna Nook (AWAC S1L), 50 days of logged data were lost from a deployment. This followed an unsuccessful 24 hour search to recover the instrument during the scheduled service trip on 29 th May This AWAC was again off position from January/February On this occasion the instrument was located nearby but was buried and could not be recovered and resulted in a loss of data from 14 th January 15 th March Monitoring at Chapel Point was suspended in the summers of 2007 and 2008, to allow beach renourishment works. Further suspension to logging occurred from May 14 th October 2008 due to these renourishment works but also because of Lynn and Inner Dowsing windfarm service boats and the need to use divers to recover the instrument in April/May 2007 due to the marker buoy being cut adrift. In addition, there was a further data gap at the site from 23 rd June 14 th July 2009 due to a power fault on the instrument. AWAC S4L at Skegness was close to cabling running to the windfarm during its construction and the site was recovered from 27 th April 29 th November On 6 th January the instrument was pulled up and left inverted, valid sea level data was obtained from the pressure records but wave data was lost. The site was suspended due to further cabling works from 15 th April 13 th October There was a loss of data in December 2008 due to instrument failure and the AWAC was recovered again in January 2009 due to cable repair work by windfarm contractors. Over the three year period, S4L had a 50% data return and was the most disrupted site. (Gardline, 2010). 1.7 Instrument locations Instrument Latitude Longitude Depth S1L Donna Nook ' N ' E 5m S2L Mablethorpe ' N ' E 5m S3L Chapel Point ' N ' E 5m S4L Skegness ' N ' E 5m LWB1 West Silver Pit ' N ' E 19m Table 1: Deployment locations between of Lincolnshire instruments. AWAC S1L was located on the flats south of the Humber channel and to the north of Donna Nook. S2L was offshore of Theddlethorpe and landward of the channels such as Theddlethorpe 4 Sea state report Lincolnshire ACMVII

11 Overfalls. The S3L and S4L AWACs were situated on a shallow flat, inshore of the Inner Dowsing bank. The LWB1 buoy is located just inshore of Silver Pit in the north of the region, approximately 26 km offshore of the northern Lincolnshire coast. Deployment sites and the onshore frontage and defences are described in the previous seas state reports (see Environment Agency, 2009; Environment Agency 2010). Figure 2: Map showing location of Lincolnshire instruments Sea state report Lincolnshire ACMVII 5

12 2 Wave statistics This section presents the key wave parameters in the annual time series that helps to describe sea conditions at the instrument location. This shows the general pattern of waves represented by a spectrum of waves of different frequencies, heights and directions through statistical measurements, time series and averages over the duration of the year. This section also includes the comparison and association of measured datasets, such as the comparison of wave height and wave period, and wave spectra plots. This analysis identifies consistency in the data, trends, quality and a summary of the data recorded. A description of all of the parameters can be found in the glossary section. 2.1 Monthly and annual means Mean significant wave height (Hs) Month Site S1L Donna Nook S2L Theddlethorpe S3L Chapel Point. S4L Skegness LWB1 West Silver Pit Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept Table 2: Monthly mean significant wave heights Hs values (m) in Year 3. Mean significant wave height (Hs) Year Site ( ) ( ) ( ) S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 3: Annual mean significant wave heights Hs values (m) for all years. Mean maximum wave height (Hmax) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 4: Monthly mean maximum wave heights Hmax values (m) in Year 3. 6 Sea state report Lincolnshire ACMVII

13 Mean maximum wave height (Hmax) Year Site ( ) ( ) ( ) S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness Table 5: Annual maximum significant wave heights Hs values (m) for all years. Mean peak wave period (Tp) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 6: Monthly mean peak wave period (Tp) values (s) in Year 3. Mean peak wave period (Tp) Year Site ( ) ( ) ( ) S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 7: Annual mean peak wave period (Tp) for all years. Mean wave period (Tz) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 8: Monthly mean wave period (Tz) values (s) in Year 3. Sea state report Lincolnshire ACMVII 7

14 Mean mean wave period (Tz) Year Site ( ) ( ) ( ) S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 9: Annual mean wave period (Tz) values (s) for all years. Mean wave direction (Mdir) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 10: Monthly mean wave direction (Mdir) values ( ) in Year 3. Median wave direction (Mdir) Year All years Site ( ) ( ) ( ) ( ) S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 11: Annual median of main wave direction (Mdir) values ( ) for all years. 2.2 Significant wave height The mean significant wave height across the three-year monitoring period is approximately 0.6 m. The instruments showed good alignment in recorded values along the coast and a good correlation to the offshore wave heights. High north-easterly waves are seen in all instruments and the effects of the wide shelf and disruptive influence of the currents and overfalls are evident. The overall mean wave height value is influenced by a stormier second year of monitoring and severe storm events in November 2007 and March This is reflected in the highest significant wave height of 0.9 m being recorded at Chapel Point in both these months. The gaps in data recording also affect overall mean calculations, especially for the Skegness AWAC where recording was interrupted by construction of an offshore windfarm and beach renourishment activities. The section of coast from Theddlethorpe to Chapel Point seems to be the most exposed section as the instruments here logged the highest waves in storm events and recorded the highest wave energy. The Donna Nook AWAC (S1L) is the most sheltered instrument and recorded the lowest wave heights. The storm thresholds were established in Year 1 based on the year of data and historic data. The threshold level has remained unchanged in order to compare storminess. In review of the three year dataset collected, the threshold, designed to capture several storms within the year 8 Sea state report Lincolnshire ACMVII

15 could be set lower, at around 2 m at each site. However the thresholds initially set for this three year monitoring period are 2 m at Donna Nook and Skegness, 2.4 m at S2L Theddlethorpe and at Chapel Point the level for AWAC S3L is 2.3 m. These threshold levels are denoted on figures 3-6 below as continuous red lines. There were a number of storm events that can be identified in all the instruments and next step will be to refine thresholds to associate them with observed onshore impacts, such as high sea spray, overtopping or beach material loss. This will allow use of wave data to assist Flood Forecasting operations, emergency works and management planning. Figure 3: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S1L at Donna Nook (blue) and the West Silver Pit wavebuoy (LWB1). The horizontal red line denotes the 2.0 m storm threshold. Sea state report Lincolnshire ACMVII 9

16 Figure 4: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S2L at Theddlethorpe (light blue) and the West Silver Pit wavebuoy (LWB1). The horizontal red line denotes a 2.4 m storm threshold. Figure 5: Plot of significant wave heights (Hs) from October 2006 to September 2009 for the AWAC S3L at Chapel Point and the West Silver Pit wavebuoy (LWB1). The horizontal red line denotes a 2.3 m storm threshold. 10 Sea state report Lincolnshire ACMVII

17 Figure 6: Plot of significant wave height (Hs) records from October 2006 to September 2009 for the AWAC S4L at Skegness and the West Silver Pit wavebuoy (LWB1). The horizontal red line denotes a 2.0 m storm threshold. Sea state report Lincolnshire ACMVII 11

18 2.3 Wave direction Figure 7: Main wave direction (Mdir) and occurrences for Lincolnshire AWACs and the offshore waverider buoy, from October 2006 to September Figure 7 shows the main wave direction for the Lincolnshire coast. In Year 3 the mean wave direction is shown to be north-easterly. Over the three years, the prominent main wave directions were observed to be north-easterly to easterly. The north-easterly waves have the longest fetch and therefore greatest energy, the coastline orientation is more exposed to these waves and they contribute to an erosion trend and southerly sediment drift from Theddlethorpe 12 Sea state report Lincolnshire ACMVII

19 to Gibraltar Point. The high storm waves are all associated with the north easterly wave direction. Donna Nook and Theddlethorpe instruments recorded a higher occurrence of easterly waves and the Skegness instrument recorded a number of southerly waves, especially in the first year of monitoring in 2006/ Peak period Peak period is also known as the dominant wave period and it describes the frequency with the highest energy. The logged data shows a very similar distribution of wave periods in Year 3 as in the previous years. The majority of waves recorded along the Lincolnshire coast are between 2-5 seconds, especially at Theddlethorpe were there is a clear spike in waves of around 4 seconds. These short period waves are wind driven and under 2 m in height. In each year a slight second peak of 8-11 second period waves can be observed. This distribution of waves have low to moderate significant wave heights, and are best seen in the subplots of S2L and S3L, due to these sites receiving the stormiest waves. The tail in each of the plots below show the recorded longer wave periods, those associated with swell waves that have a flatter shape and occur during persistent and stronger winds from offshore. These waves are few, but long period waves of over 15 seconds that would be associated with distant offshore storm conditions are present. Figure 8: A distribution plot of wave periods for each AWAC (clockwise from top left: S1L, S2L, S3L, S4L) from The highest occurrence of wave periods is just under 5 seconds, with a noticeable second peak at around 9 seconds. The disruption of monitoring at S4L is reflected in the number of occurrences of each wave. Sea state report Lincolnshire ACMVII 13

20 Figure 9: Occurrence of Peak wave periods recorded by LWB1 West Silver Pit from September 2006 October Period and significant wave height Plotting wave period (Tp) and significant wave height (Hs) are of value in determining heights that can be expected for various distributions of waves and therefore applied to defence overtopping studies and in determining beach response to wave loading. Figure 10 (overleaf) represents the region s wave climate and shows generally the greatest concentration of waves are of short 3-6 second periods and under 1 m in height. The dataset shows a monthly concentration of waves under 0.6 m at these periods, which supports the average wave height derived from the significant wave height records. The three year plots below show the spread of wave heights at various periods. In stormier months the relationship between moderate wave periods and increasing high wave heights is seen. The highest wave heights are associated with a peak wave period of around seconds. Waves generated locally by wind are seen to have short periods and wave heights just over 1 m. In annual plots swell waves with a longer wave period are more noticeable. These waves are under 0.5 m in height with peak periods extending up to 20 seconds. The height to which a wave will rise as it enters shallow coastal waters is not only dependent on its period but is influenced by the bathymetry of the area; the water depth and beach slope. At Lincolnshire wave heights are shown to attenuate as they travel over the gently sloping bathymetry. The presence of sandbanks can also cause the offshore waves passing West Silver Pit buoy to slow, rise and even break. In addition to the bathymetry, interaction with the strong current influence the incoming waves are the most likely reason for the short wave periods and heights observed. 14 Sea state report Lincolnshire ACMVII

21 Figure 10: Scatter plots indicating joint distribution of Hs and wave period for October 2006 September 2009 at each AWAC site (clockwise from top left: S1L, S2L, S3L and S4L). The coloured points shows wave periods with the highest energy (Tp) against significant wave height (Hs) and the black shows the mean wave period (Tz) against Hs. 2.6 Wave energy Wave energy is analysed for frequency bands carrying the most energy out of the 64 bands recorded by the AWAC onboard data logger. The spectra data supports the results from individual wave parameters such as a dominant north-easterly wave direction but also the association of these components. This is demonstrated in the spectra plot of the surge event at Chapel Point in November 2007 (Year 2) (Figure 11). The plot shows the spread of energy (m 2 /Hz) across different wave periods, below this is a plot of the corresponding coming wave direction at each coincident wave period, and below this, in the third plot is the wave height. During periods of low wave height, wave energy is seen to be negligible across all wave periods. Only very short period waves are present under these conditions. The wave direction plot shows the choppy nature of these very short period waves. They are quick to react to changes in wind direction and are comprised of indistinguishable wave trains and very locally generated waves from wind forcing. However as wave heights increase there is a clear association with wave energy. Waves generated away from the AWAC and the coastline have a greater fetch for energy to be transferred to the wave. It can also be seen that the highest energy is not contained in the longest wave periods, but in the centre of the plot, at periods ranging from 8-14 seconds. Within storm events there is also an observed transfer of peak energy, at the start of an event the peak energy is seen in the shorter wave periods, but over time and as the storm persists the peak energy moves to the longer wave lengths. This is because the shorter period waves are the first waves of the event to arrive at the instrument. At Chapel Point in November 2007 the high energy is associated with the wave periods around 14 seconds and with heights of over 2 m. Sea state report Lincolnshire ACMVII 15

22 Figure 11: Wave spectra plot showing for S3L at Chapel Point for the month of November The top plot shows high energy (up to 2 m 2 /Hz) waves, coloured in red and low energy waves in blue. The centre plot shows the corresponding wave directions, which are coloured according to their coming direction in degrees. Northerly waves coming from around 0 or 360 are either dark red or dark blue, southerly waves in turn are green. Peaks in wave heights are shown to occur during high energy periods and from a north easterly direction Average Energy (sq.m/hz). Averaged Wave Energy Distribution per Frequency S1L - 85% S2L - 99% S3L - 80% S4L - 50% Wave Period (seconds) Figure 12: Wave energy and wave period distribution over frequency for all years ( ), based on QA ed return data, shown as a percentage. The exposed nature of the Lincolnshire coast means it is impacted by high energy waves. Across the three-year period, the stormy second year of monitoring is visible in the below plots of wave energy. The storms in March and the surge in November meant there was an increase in high energy waves on the Lincolnshire coast that year. Donna Nook is shown to be the most sheltered site and least exposed to these waves. The results at Skegness logged by S4L are again affected by the months of suspended monitoring and the associated conditions that were not recorded during these times. 16 Sea state report Lincolnshire ACMVII

23 5 Annual Averages of Wave Energy (m2/hz) in all Spectral Frequency Bands year 1 year 2 year L 2L 3L 4L Figure 13: Comparison of the average wave energy (Tz) at each AWAC site for each year ( ) Annual Averages of Wave Energy (m2/hz) at the Peak Spectral Frequency Band year 1 year 2 year L 2L 3L 4L Figure 14: Comparison of the peak wave energy (Tp) at each Lincolnshire AWAC for each year ( ). Sea state report Lincolnshire ACMVII 17

24 2.7 Wave return periods Return periods are an indicator of the frequency a wave can be expected from a certain height and direction sector. The values below are the frequency of a particular event occurring in years. Therefore the lower the number the less frequent the event. For example a return period of 0.25 would mean the event would occur every 0.25 years or 91 days and a return period of would occur once every 1.8 days. It is of course possible that an event with a frequency of every 2 days will occur on successive days, and does not mean an event will occur only at these intervals. Direction Significant wave height (Hs) (m) band band Total Total Table 12: Return period for S1L AWAC ( ). Direction Significant wave height (Hs) (m) band band Total Total Table 13: Return period for S2L AWAC ( ). 18 Sea state report Lincolnshire ACMVII

25 Direction Significant wave height (Hs) (m) band band Total Total Table 14: Return period for S3L AWAC ( ). Direction Significant wave height (Hs) (m) band band Total Total Table 15: Return period for S4L AWAC ( ). Sea state report Lincolnshire ACMVII 19

26 3 Temperature A thermometer is located within the AWAC instrument, which is mounted in a frame and this sits on the bed. Therefore the temperature values below are recorded at a depth of approximately 5 m CD. The wavebuoy is a measure of sea surface temperature. There is an unsurprising seasonal pattern in the temperatures recorded, with a rise in temperatures through spring and peaking in August. At Skegness AWAC S4L, the highest temperature of 18.1 c was recorded. Temperatures then start to decline in November, falling to a low of 3.6 c at Chapel Point in February. This pattern of warmer water in August and coldest in February is observed in the full three year dataset. Temperature ( c) Month Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 16: Monthly mean temperatures. October 2008 September Temperature ( c) October December February April June August October December February April June August October December February April June August S1L S2L S3L S4L Month ( ) Figure 15: Lincolnshire monthly mean temperatures. All sites follow the same temperature pattern. Note temperature readings are taken at the sea bed. 20 Sea state report Lincolnshire ACMVII

27 4 Sea level & tides This section details sea level in Year 3, and provides tide levels at each site and the Class A tide gauge at Immingham. The water levels do not vary significantly across the frontage, with a difference in Highest Astronomical Tide of 20 cm between Donna Nook and Skegness in the year. Skegness has the highest tide level and tidal range of the Lincolnshire AWACs. Sea levels have been consistent across the three year period with the highest mean values being recorded in October and November. Sea level (modn) Month Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug S1L Donna Nook S2L Theddlethorp e S3L Chapel Point. S4L Skegness Sep t Table 17: Monthly mean sea levels relative to Ordnance Datum Newlyn for Year 3 (October 2008 September 2009). Lincolnshire tide levels (modn) Site HAT MHWS MHWN MLWN MLWS LAT Mean HW interval (relative to Immingham) Range on Springs (m) Immingham hrs 32 mins 5.97 S1L Donna Nook S2L Theddlethorpe S3L Chapel Point. S4L Skegness hrs 5 mins (- 27 mins) 5 hrs 16 mins (- 16 mins) 5 hrs 31 mins (- 01 mins) 5 hrs 40 mins (+ 08 mins) Table 18: Tidal parameters derived from Gardline Environmental s 60-constituent harmonic analysis of the third year of AWAC data Sea state report Lincolnshire ACMVII 21

28 Figure 16: Sea level at each AWAC (S1L, S2L, S3L, S4L) for October 2008 September The red lines denote the Flood Forecasting minimum trigger levels for the region based on levels at the Immingham tide gauge 5. The Flood Alert trigger level at Immingham is 3.3 m, Flood Warning is 4.2 m and the Severe Flood Warning level is 4.6 m. 22 Sea state report Lincolnshire ACMVII

29 5 Storminess & extremes 5.1 Storm wave events Analysis of storm wave events and frequency allow us to infer patterns of long term trends which may reflect change in climate. From analysis of extreme values we can determine how the pattern of wave characteristics in calm waters, such as direction and height, change in storm conditions. However, storm generated wave directions may vary across an area of sea and on their approach to the coast after being recorded by an offshore buoy. Within this series of reports a storm event is defined according to the Beach Management Manual (CIRIA, 1996). Using the peaks over threshold method of significant wave heights (Hs) that exceed a defined threshold. This value is based on historic monitoring of storm events in the Anglian region and is expected to become more accurate as a result of these three years of monitoring. Suspected storms are also identifiable in wave spectra plots (such as Figure 11). The duration of a storm is considered to be 16 hours around the peak wave height (Hs). 5.2 Monthly wave maxima Maximum significant wave heights Hs (m) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West 4.6 Silver Pit Table 19: Monthly maximum significant wave height (Hs) values(m) Maximum wave heights Hmax (m) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 20: Monthly maximum wave heights (Hmax) values (m) Sea state report Lincolnshire ACMVII 23

30 Maximum peak wave period Tp (s) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 21: Monthly maximum peak wave period values (s) Maximum mean wave period Tz (s) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 22: Monthly maximum mean wave period values (s) 5.3 Monthly temperature maxima Maximum temperature ( c) Month Site Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sept S1L Donna Nook S2L Theddlethorpe S3L Chapel Point S4L Skegness LWB1 West Silver Pit Table 23: Monthly maximum temperature ( c). The AWAC temperature is a measurement at bed depth, the DWR record is a sea surface temperature measurement. 5.4 Highest Lincolnshire storm event 2008/2009 There were eight storms recorded in the third year of monitoring and marked as waves crossing a threshold in at least one of the AWAC locations. These events are shown on the Storm calendar in Figure 18. The highest storm of the year was recorded at Chapel Point on the 28 th March 2009 with a 3.05 m wave height. In each year of monitoring the highest wave has been recorded at Chapel Point and in the month of March. In March 2007 a 3.01 m wave was recorded and in March 2008 the highest wave height of all years of 3.52 m was logged. 24 Sea state report Lincolnshire ACMVII

31 Date/Time 28/03/ :00 (S3L) 01/02/ :00 (S2L) 02/02/ :00 (S2L) H s (m) H max (m) T p (s) T z ( ) Dir. ( ) Water depth (m) Water level elevation (modn) Residual (m) Table 24: The three highest storm wave (Hs) events, and associated parameters. The 3.52 m highest wave, occurred on the 22 nd March Strong northerly winds over the 21 st and 22 nd March led to waves crossing the threshold level on successive High Waters but peaking at 05:00 on the morning of the 22 nd. Wave heights of 3.9 m were recorded at the Inner Dowsing wavebuoy an hour prior. The Chapel Point wave had a 9.3 second peak period, although 12 second period waves were recorded at the same time to the north at Theddlethorpe. 5.5 Surge events Tides are not the only variable affecting water levels along the Anglian coast. Any contributing factors that are not part of the astronomical tide, such as non average weather conditions, will have an impact. An area of low pressure and a storm can produce large scale tides know as storm surges that propagate through the North Sea. Strong winds also generate high energy waves. The addition of high waves and/or a surge combined with a high astronomical water level can severely impact on coastal defences, and risk overtopping defences and flooding. A surge or residual is regarded as the difference of the sea level, calculated from historical sea levels and the theoretical harmonic tidal curve, and the actual sea level recorded by the AWAC. The reasons why a water level differs from the usual or predicted can be due to a number of causes, such as changes in air pressure or winds. This report highlights significant surge events, but does not seek to explain why there is a difference or isolate the principle components of a surge. Surges are identified based on the 60-constituent harmonic analysis, carried out by Gardline Environmental, which has removed the tidal influence, from the residuals (Gardline, 2008). The UK Met Office s classification of surge event is when the residual exceeds 0.6 m at two or more tide gauges. This report will detail significant surge events where the residual is greater than 1 m. The surge has to be visible in neighbouring instruments to be considered significant and to eliminate instances of instrument error. 5.6 Highest Lincolnshire surge events 2008/2009 Storm surge event date Residual (m) Water level elevation (modn) 20/12/ /11/ /01/ /09/ HW level elevation (modn) HW time & difference to peak surge 08:00 (+3:00 hrs) 22:55 (+4:10 hrs) 13:20 (+3:50 hrs) 11:40 (+2:15 hrs) H s (m) T p (s) Dir. ( ) Table 25: Surge events (residuals greater than 1 m ODN) in Year 3. Listed in rank of highest residual surge recorded. The time and height of the highest surge value of an event is listed. All the highest surges where recorded by the AWAC S4L at Skegness. The related coincident water level and hourly wave data Sea state report Lincolnshire ACMVII 25